It is known in the art of fiber optics that Bragg gratings (i.e., periodic or aperiodic variations in the refractive index of the optical fiber) exhibit a predetermined wavelength reflection profile. As is known, a fiber Bragg grating is the result of a photo-refractive effect. In particular, when the core of a photosensitive (e.g., germania-doped) optical fiber is exposed to ultra-violet radiation in a predetermined wavelength range, a permanent change in the refractive index is produced. The magnitude of the refractive index change is related to the intensity of the incident radiation and the time of exposure.
As is also known, a Bragg grating is impressed (or embedded or written or imprinted) into the core of an optical fiber by allowing two coherent nominally plane optical waves to interfere within the fiber core at a location along the fiber where the grating is desired. The resulting interference pattern is a standing wave which exists along the longitudinal axis of the fiber having an intensity variation which causes a corresponding spatially periodic or aperiodic variation in refractive index along the longitudinal axis of the fiber. For periodic variations, the grating has a peak reflection wavelength related to twice the spatial period (or grating spacing). The spatial periodicity or other spatial variations of the refractive index of the fiber, and the resultant reflectivity profile, are a function of the wavelength, amplitude, and/or phase of the two incident writing beams and/or their angle of intersection within the fiber.
The above described techniques are described in U.S. Pat. Nos. 4,807,950 and 4,725,110, entitled "Method for Impressing Gratings Within Fiber Optics", both to Glenn et al and U.S. Pat. No. 5,388,173, entitled "Method and Apparatus for Forming Aperiodic Gratings in Optical Fibers", to Glenn, which are hereby incorporated by reference.
For some applications, it is desirable to write a grating with a high intensity interference pattern. For example, a standard grating or "Type 1" grating, is typically written with either a standard intensity (about 50-500 mjoules/cm.sup.2) or high intensity (about 500-800 mjoules/cm.sup.2) light. For a Type 1 grating, the higher the intensity of the light, the faster the grating is written and, in general, the higher the resultant reflectivity. Also, a high temperature grating or "Type 2" grating is written with very high light intensities (about 0.9-1.5 joules/cm.sup.2 or higher), such as is discussed in J. L. Archambault et al., "100% Reflectivity Bragg Reflectors produced in Optical Fibers by Single Excimer Laser Pulses", Electronics Letters 29(1993) pp 453-455. However, with a high intensity light (e.g., greater than about 800 mjoules/cm.sup.2 for average surface quality glass), the electric field at the air-to-glass interface at the surface of the fiber would be so high that it would cause ablations (i.e., melting, evaporation, or particle discharge) at the surface of the fiber which cause damage to the outer surface of the fiber and reduce the tensile strength of the fiber. The maximum intensity depends on the glass surface quality, e.g., the amount of contamination, such as dust, dirt, oils, etc. Thus, for poor surface quality glass the maximum intensity may be lower than 800 mjoules/cm.sup.2 (e.g., 500 mjoules/cm.sup.2 or lower). The cleaner the glass, the higher the intensity that the glass can withstand before surface damage occurs. Thus, it would be desirable to be able to write gratings with such high intensity light without damaging the fiber outer surface.